High image quality and high speed xerography requires imagers that can expose the photoreceptor with high resolution. A multi-beam raster optical scanner (ROS) provides a technique to achieve higher addressability and throughput. Multi-beam ROS's simultaneously sweep many beams across the photoreceptor and these beams can be more closely spaced than print engines with one beam. However, it is difficult to manufacture multi-beam ROS's with uniform beam spacing and exposure. The variation in beam spacing and exposure leads to differential development from each beam and objectionable high frequency banding.
In electro-photographic marking, a latent image is created by selectively discharging a photoreceptor. One way to selectively discharge a photoreceptor is to sweep a beam repeatedly across the photoreceptor as it moves relative to the beam. An image is written by turning the light on and off in the desired pattern. The image will be subsequently developed with toner and transferred to paper.
For an imager that sweeps a single beam across the photoreceptor, the addressability in the cross process direction is determined by how fast the beam can be turned on and off. The addressability in the process direction is determined by how far the photoreceptor moves between sweeps of the beam.
Image quality can be improved by increasing the addressability of the imager. Specifically, 2400 spots per inch (spi) images can be of higher quality than 600 spi images. Higher addressability will improve uniformity and density stability of the highlights and midtones. High addressability images will be more robust to marking process variations. A tone reproduction curve (TRC) may have less contouring and there can be more flexibility in a halftone design.
One way to achieve high addressability is to sweep more than one beam simultaneously across the photoreceptor. One way to generate multiple beams is through use of a vertical cavity self emitting laser (VCSEL). A VCSEL ROS consists of an array of diode lasers that emit light vertically from the chip. The beams can be focused so that they will sweep a series of parallel swaths across the photoreceptor.